Traditionally, a distance measuring device that transmits a frequency-swept electromagnetic wave to an object to be measured and measures the distance between the distance measuring device and the object using a wave reflected on the object has been developed. For example, there is a method for receiving a reflected wave while continuously changing the frequency of a wave such as a millimeter wave or a microwave, estimating a time period for the reciprocation of the reflected wave based on a frequency difference (beat frequency) between the transmitted wave and the reflected wave, and calculating a distance corresponding to the time period. This distance measurement method is also referred to as frequency modulated continuous wave (FMCW) scheme (refer to Japanese Laid-open Patent Publication No. 2000-055716, Japanese Laid-open Patent Publication No. 2007-078518, and Japanese National Publication of International Patent Application No. 2010-528317).
A distance measurement method such as the aforementioned FMCW scheme assumes that an object targeted for distance measurement is in a stationary state (or quasi-stationary state). Thus, when the object targeted for the distance measurement is moving, it is hard to improve the accuracy of analysis due to an effect of a Doppler shift component. Especially, in the measurement of a water level such as a water surface or a liquid surface, a distance measurement error easily occurs due to the fluctuation of a liquid surface and it is difficult to measure the water level with high accuracy.